When Cisco officials disclosed the bug last week in a range of Adaptive Security Appliance products, they said they had no evidence anyone was actively exploiting it. Earlier this week, the officials updated their advisory to indicate that was no longer the case.

"The Cisco Product Security Incident Response Team (PSIRT) is aware of public knowledge of the vulnerability that is described in this advisory," the officials wrote. "Cisco PSIRT is aware of attempted malicious use of the vulnerability described in this advisory."

The update didn't say how widespread the attacks are, whether any of them are succeeding, or who is carrying them out. On Twitter on Thursday, Craig Williams, a Cisco researcher and director of outreach for Cisco's Talos security team, wrote of the vulnerability: "This is not a drill..Patch immediately. Exploitation, albeit lame DoS so far, has been observed in the field."